I ended up buying the DT 880 on eBay for a much better price than I could find anywhere else ($145). Although they are used, they appear to be in excellent condition. The reviews here and on Amazon, as well as the comments in this thread, led me to the (hopefully correct) conclusion that they would be better for me than the HD 598 and Q701, and most people find them to be extremely comfortable. As a bonus, the listing was for the DT 880 Pro model, but the pictures definitely were of the more expensive DT 880 Premium model. I realize they are basically the same, but the reduced clamp pressure of the Premium is a nice bonus. Thanks to everyone who posted advice.
Edit: This was such a great purchase for me. For just $5 more than I paid for the SRH840, I will finally be able to compare it with another quality pair of headphones. If I don't like them, I can sell them on Amazon and possibly even gain money in the process. It feels great to come into a topic with only 2 options and come out with a third, most likely better, option for even a little less money.
You might have to buy new Velour ear pads (EDT 990 V) for the DT880.
I had to buy new pads for my used DT880 Premium 600-Ohm, (but the price for the used DT880s was very fair).